The present invention relates generally to the manufacturing of blanks from sheet material, such as blanks for folded boxes, labels and the like. More particularly, the invention relates to a method of stamping out sheet material blanks and to an apparatus for carrying out the method. Still more particularly, the invention relates to such a method and apparatus for carrying out the stamping of sheet material blanks in a continuous manner.
The manufacture of sheet material blanks by stamping them out of sheet material is well known, and it is also known how to do this continuously. The prior-art devices include a type wherein a cutting or stamping roller is provided that has cutting edges on its circumference and cooperates with a counter roller. The two rollers form with one another a nip and the sheet material from which the blanks are to be stamped is moved in a straight line through the nip so that the cutting edges on the cutting roller cut through the sheet material all the way into engagement with the hard surface of the cooperating counter roller. Because of the straight line movement of the sheet material through the nip of the two rollers the surface of the sheet material comes in contact with the cutting edges only when the same engage it and press it against the surface of the cooperating counter roller. The stamping-out of blanks from the sheet material (whether the same be of single-sheet configuration or in form of an elongated web or strip) progresses in direction opposite to the advancement of the sheet material; i.e. the respectively leading portion of the sheet material is always cut apart as the blanks are stamped from it, whereas the portion or portions of the sheet material which are approaching the nip between the rollers have not yet been touched. During the actual cutting operation, that is at the time at which the cutting edges penetrate into the sheet material, the speed of movement of the cutting edge tip does not equal the speed of advancement of the sheet material at the point of contact thereof with the cutting edge tip, due to the rotary movement of the cutting roller. These differences in the speed may be minor, but they are sufficient to assure that during the penetration of the cutting edge through the sheet material there will be sufficient relative movement of the sheet material and the penetrating cutting edge to cause crinkling or otherwise deforming of the sheet material along the cut that is being made.
Another type of prior-art apparatus utilizes the same principle of a rotary cutting roller having cutting edges and a hard-surfaced counter roller, but in this construction the sheet material from which the blanks are to be stamped engages the circumference of the counter roller and is supplied to the nip between the two rollers in an arcuate path rather than in a straight-line path. The two rollers are connected by a set of gears to assure that they are rotated in such a manner that the surface of the counter roller and the tips of the cutting blades will have identical angular velocities. In such a machine that side of the sheet material from which the blanks are to be stamped which engages the surface of the counter roller travels at the angular velocity of this surface; hence, since the cutting edges cut through the sheet material into engagement with the surface of the counter roller the same problems will occur which have been described above with the other type of prior-art equipment.
Moreover, these two types of prior-art apparatus have further disadvantages beyond those which have already been outlined.
It has been found that it is only theoretically possible to obtain a 100% severing of the sheet material blank from the surrounding sheet material with this type of equipment. Due to the differential wear of the cutting edges, nicks in the cutting edges, manufacturing tolerances of the cutting roller having the cutting edges, out-of-round configurations of the two rollers, deformations of the rollers and surface damage to the counter roller, to name the most frequent occurrences, it is in actual practice impossible to obtain a 100% severance of the blanks from the sheet material. This means that because of this the blank will continue to adhere to the surroundng sheet material by way of fibers which have not been fully severed and which interfere with the subsequent separation of the blanks from the sheet material.
Another disadvantage is that there must be a prestress in the operation of this type of machine, that is a force at which all of the cutting edges that are in engagement with the sheet material at any one time are pressed against the sheet material and against the hard surface of the counter roller. Only in this manner is it possible to obtain any severing at all. However, this prestress means that the cutting edges engage the hard surface of the counter roller with sufficient force to undergo rapid wear, especially insofar as the longitudinally extending cutting edges are concerned which wear away more rapidly than the transversely extending ones, it having been found that the specific forces which act upon the longitudinally extending cutting edges are higher than those acting upon the transversely extending ones. This wear of the cutting edges requires correction by way of an increase of the prestress in order to obtain a cutting of the edges through the sheet material despite the wear. The increase in the prestress, however, leads to a further increase in the wear. This cycle is repeated until, after a very short period of time, the cutting edges are so blunt that they must be renewed since any further increase in the prestress will not cause them to penetrate through the sheet material any longer. Finally, this type of equipment also requires devices for increasing the prestress to compensate for wear.
Another type of prior-art apparatus uses a cutting roller as mentioned before, but employs a counter roller having a relatively soft and yieldable surface. In this case, the cutting edges penetrate the sheet material and sever it at their point of contact with the yieldable surface of the counter roller, pressing the sheet material against and to some extent into the yieldable surface of the counter roller until the resistance of the surface to further deformation becomes so great that the actual severing of the sheet material occurs.
With this type of equipment it is theoretically possible to obtain a 100% cut-through of the sheet material. The yielding of the surface of the counter roller tends to compensate for the differential wear of the cutting edges, and also for manufacturing tolerances and out-of-round conditions of the two rollers. However, practical experience has shown that nicks and similar flaws in the cutting edges are not compensated-for so that the presence of residual fibers which connect the blank with the surrounding sheet material scrap, continues to remain a problem even in this type of equipment. Moreover, such fibers also remain when the restoring force of the yieldable surface of the counter roller is not sufficient to press back hard enough against the cutting edges to assure that the latter penetrate relatively harder portions of the sheet material, for instance a small wood chip that might be imbedded in the sheet material. The problems outlined above with respect to the separation of the blanks from the surrounding sheet material scrap therefore continue to exist in this type of equipment, as they do in the earlier-mentioned machines. Moreover, during the actual cutting operation the cutting edge tends to press the sheet material so far into the elastically yieldable surface of the counter roller until the resistance of this surface to yielding has become high enough so that the actual cutting operation will take place. This, however, produces an unclean cut (e.g. a fuzzy or crinkly edge) which makes it impossible to use this type of machine where sharp clean edges are required to be produced on the blank.
In all of this equipment that has been mentioned the sheet material always has the blank or blanks stamped out at its leading regions, whereas the trailing regions (the ones which are approaching the nip between the rollers) have not yet been touched by the cutting edges. The differences in the angular speed between the cutting edges and the advancing material cause the aforementioned formation of ruffles or the like at the cut edges. After the cut the cutting edges immediately are withdrawn from the sheet material and move away from the same. For this reason there is no guarantee that all blanks will be of exactly identical size because not all of the cutting edges participating in the cutting or stamping-out of a blank penetrate the sheet material at one and the same time. The fact that the sheet material approaching the nip between the rollers is as yet untouched, whereas in the nip and downstream of the nip the sheet material has already had blanks stamped out of it means that the structural integrity of the sheet material is highly unstable so that, as the sheet material with the almost but not completely severed blanks is subsequently advanced into a device for pushing the blanks out, great difficulties are experienced in guiding the sheet material into this device, especially where the operation takes place at high speeds as is always the case with rotating cutting rollers, and where large sheet-material formats are involved.